"I have great trouble saying no to people, and at times it makes me miserable. Why is it so hard?"
It's such a little word, just two letters, but not being able to say it can be downright dangerous, and can also quickly make life unmanageable. Even if you're safety isn't at risk, too many "yes when you mean no" moments leads to resentment: the wish to be agreeable is eclipsed by exhaustion, and the growing pain of our own needs and wishes becomes ignored.
The real problem though is when our belief in our own value is so skewed towards what we do, rather than who we are. That's when the possibility of disappointing others takes on an extra dimension: What if they don't like me any more?
Childhood, schooling, and our work-obsessed culture can all convince us that our value and sense of self lies in what we do. To be useful is to be valuable. The problem with this is it means our self-esteem, our regard for our self, is conditional. Conditional on what we can achieve, and what we can do for others.
Learning to say no then is not just about practicing the use of the word, although that can be useful: one strategy is to practice saying no to things that don't really matter.